Focused and directed laser beams are commonly used for a variety of processes, such as drilling of blind, through and micro-vias, laser imaging, dicing of substrates and modification or customization of integrated circuits, drilling, cutting, and selective material removal and other complex machining and micro-machining operations involving materials such as metals, polymers, integrated circuits, substrates, ceramics and other materials. Such processes have become very complex, often involving the concurrent or sequential of use of single or multiple lasers or multiple types of lasers, such as visible, infra-red (IR) and ultraviolet (UV) lasers, in concurrent or sequential operations. In generally all such laser processes, however, the general object of a laser system is to controllably and reliably direct, focus and concentrate the energy of one or more laser beans to converge each beam at a desired spot or to image an apertured area of a laser beam onto the surface of an object.
A number recurring problems of conventional laser systems of the prior art, however, directly affect the reliable and controllable “pointing” of a laser beam to a desired location. The first, which is illustrated in FIGS. 1A and 1B, is often referred to as “beam wobble” or “pointing instability” and is the radial deviation of the Beam Axis 10 a Laser Beam 12 from an Optimum Centerline 14 by a Deviation Angle θ and is often related to variations in the pulse energy of the laser beam, which is often referred to as “pumping jitter”. Pointing instability is essentially inherent in both the properties of a Laser 16 itself and in the normal operations of a Laser 16, such as “pumping jitter”.
A second problem of the prior art is illustrated in FIGS. 2A and 2B and is often referred to as “thermal drift”, which again causes the Beam Axis 10 of a Laser Beam 12 to drift from an Optimum Centerline 14. Thermal drift is generally regarded as due to changes in the parameters of the Laser 16 due to changes in the laser duty cycle, heating during operation, changes in power levels of the Laser 16. It should be noted that, unlike “pointing instability” which results in an angular deviation of the Beam Axis 10 from the Optimum Centerline 14, “thermal drift” results in a linear radial drift of the Beam Axis 10 with respect to the Optimum Centerline 14. That is, the Beam Axis 10 of a Laser Beam 12 remains parallel to the axis of Optimum Centerline 14, but drifts radially away from Optimum Centerline 14.
Yet a third problem of the prior art is that of beam mode changes over time, which results in “hot spots”, or distortions of the beam profile. If the profile of the beam is non-uniform or does not have an optimum Gaussian profile, the shape of the profile cannot be subsequently shaped into the preferred “flat top” profile, which will adversely effect the quality of the processes performed by the laser system, such as micro-machining or the drilling of microvias. This problem is further compounded, of course, by pointing instabilities and thermal drift.
Effectively all lasers used for micro-machining, such as microvia drilling, exhibit pointing instability, thermal drift and profile distortion, and there have been many attempts to correct or at least control these problems. For example, laser systems of the prior art have attempted to correct the effects of “pointing instability” and “thermal drift” by the use of actively controlled servo-mirrors, which are controlled to redirect a laser beam so as to correct for the “pointing instability” and “thermal drift”. Such methods, however, require detecting and comparing the actual path of a beam due to pointing instability or thermal instability with the desired optimum path for the beam and controlling the servo-mirrors so as to direct the beam into the desired path. Not only are such methods complex and expensive, but they have an inherent time delay in detecting and correcting the effects of pointing instability or thermal drift, and introduce errors of their own due to mechanical and control system tolerances and have thereby not provided completely satisfactory solutions to these problems.
Other approaches of the prior art to these problems have used optical elements in the laser beam path to correct for pointing instabilities and thermal drift and to shape the beam into the optimum Gaussian and flat-top profiles for micro-machining, such as the drilling of microvias. A recurring problem, however, is that when the an optical beam shaping system is illuminated poorly, that is, either at an incident angle or with a laterally displaced beam, such as may result from pointing instabilities, thermal drift or hot spots, the optical beam shaping elements are not able to shape the laser beam into the desired profile.
The basic problems arising with the use of optical elements to correct or compensate for pointing instability and thermal drift are illustrated in FIGS. 3A and 3B with respect to the use of holographic optical elements (HOEs) and standard symmetric holographic optical element (SSHOEs) employed as beam shaping elements. FIG. 3A, for example, illustrates the results of radial displacement due to thermal drift effects in the case of a Holographic Optical Element (HOE) and, in particular, with respect to a Standard Symmetric Holographic Optical Element (SSHOE) 18, or an equivalent lens. Because the SSHOE 18 is symmetric, a Laser Beam 12A that enters the SSHOE 18 along a Beam Axis 10A that is parallel to the HOE Axis 20 will exit the SSHOE 18 as Laser Beam 12B on Beam Axis 10B wherein Beam Axis 10B is coaxial with and a linear continuation of Beam Axis 10B. More specifically, a Laser Beam 12A entering the SSHOE 18 along a Beam Axis 10A that is parallel to but radially displaced by a distance D from the HOE Axis 20 will exit the SSHOE 18 along the same Beam Axis 10A, indicated as Beam Axis 10B, and will remain radially displaced with respect to the HOE Axis 20 by a distance D. As such, a SSHOE 18 or equivalent symmetric lens will not radially redirect the Beam Axis 10 of an entering Laser Beam 12 with respect to the HOE Axis 20 of the SSHOE 18, and thereby cannot correct for or control thermal drift effects.
Referring to FIG. 3B, a Laser Beam 12A effected by “pointing instability” will enter an Entry Face 22 of the SSHOE 18 along Beam Axis 10A having an angular deviation θ with respect to the HOE Axis 20, that is, will not be parallel with the HOE Axis 20. Because of the symmetry of a SSHOE 18 or equivalent symmetric lens, however, the Laser Beam 12B will exit the Exit Face 24 of the SSHOE 18 along a Beam Axis 10B that is the continuation of the Beam Axis 10A along which the Laser Beam 12A entered the SSHOE 18. As in the case of thermal drift, therefore, conventional SSHOEs 18 and similar symmetric lenses cannot correct for or control pointing instability and the resulting angular deviation of the Beam Axis 10.
Instability and shifting in the input beams to beamshifting elements, however, can frequently lead to yet other problems in typical beam delivery systems, such as a micro-machining system for the drilling of microvias, by distortion of the laser beams generated by the beamshaping optics of the system.
For example, in a typical laser beam delivery system such as illustrated in FIGS. 4A through 4D, the Laser 12 is typically comprised of diode pumped solid state (DPSS) laser or a Diode Pumped Fiber (DPFL) laser, which tend to generate TEM00 single mode laser beams with optimum Gaussian energy profiles. In a typical system such as illustrated in FIGS. 4A through 4D, Beamshaping Optics 26 are used to reshape an Output Beam 12I having an optimum Gaussian Profile 12GP into an Output Beam 12O having a Flat-Top Profile 12FP, that is, a uniform energy profile particularly advantageous for micromachining operations, and are typically comprised of, for example, diffractive or holographic beam diffusers or shaping optics.
As is well known, the Input Beam 12I generated by a Laser 12 will tend to drift laterally by amounts ranging from a few microns to several hundreds of microns with changes in the Laser 12 parameters, such as the pump diode current changes, harmonic crystal shifts, changes in the pulsing frequency or repetition rate, and so on. The typical results of such lateral drifts of the Beam Axis 10 of a Input Beam 12I from an Optimum Centerline 14 that is coaxial with the Optical Centerline 26C of Beamshaping Optics 26 are represented diagrammatically in FIGS. 4A, 4B, 4C and 4D. FIGS. 4A and 4B illustrate the situation wherein the Beam Axis 10 of Input Beam 12I is coaxial with Optical Centerline 26C of Beamshaping Optics 26, with FIG. 4B being a superimposed comparison of the energy profiles of Input Beam 12I and the resulting Output Beam 12O. FIGS. 4C and 4D, in turn, illustrate the situation wherein the Beam Axis 10 of Input Beam 12I is laterally offset with respect to the Optical Centerline 26C of Beamshaping Optics 26, with FIG. 4D again being a superimposed comparison of the energy profiles of the input and output beams.
As illustrated in FIGS. 4A through 4D, each lateral shift of the Input Beam 12I with respect to the axis of Beamshaping Optics 26, however, that is, each shift from the situation of FIGS. 4A and 4B to the situation of FIGS. 4C and 4D, will result in a non-uniform energy profile in Output Beam 12O. In the typical situation as illustrated in FIGS. 4C and 4D, for example, a Lateral Offset 64O of Input Beam 12I will result in the generation of either or both of a “Hotspot” 64S in the energy profile of Output Beam 12O and a deficiency or “Deficiency” 64D in the energy profile of Output Beam 12O. As illustrated, a Hotspot 64S is a region of an energy profile in which the energy level is higher than desired while a Deficiency 64D is a region of an energy profile in which the energy level is lower than desired. In this regard, it should be noted that Hotspots 64S are typically formed on the side of the energy profile in the direction of the Lateral Offset 64O of the Input Beam 12I, with Deficiencies 64D typically appearing in the energy profile in the direction opposite to the Lateral Offset 64O.
Such distortions in the energy profiles of the Output Beam 12O obviously degrade the performance of a laser beam delivery system such as a laser micromaching system. As a result, each such shift of the beam axis requires either a realignment of the beam delivery system in order to restore the desired Flat-Top Profile 12F of Output Beam 12O. The required realignment of the system may be accomplished by either or both of a realignment of Beamshaping Optics 26 to the new location of Beam Axis 10 of Input Beam 12I or a realignment of Beam Axis 10 of Input Beam 12I to the Optical Centerline 26C of Beamshaping Optics 26, either of which represents a significant down time for the system. It will be appreciated that the need to realign the laser beam system optics or the laser beam for each lateral shift of the laser beam, regardless of the cause of the shift, will be a significant problem as an industrial laser system on the production floor will typically, and for example, require realignment several times a day or even several times a work shift.
In this regard, it must also be noted that while non-symmetric optical elements, including compensator and remapping elements, may be employed to address some of the problems arising from unwanted lateral shifts in a laser beam, such non-symmetric elements are typically comprised of holographic or diffractive optical elements. Such optical elements typically have fixed characteristics that, because they are fixed while the possible lateral shifts of a laser beam are not fixed, may not be able to adequately address the full range of possible lateral shifts of a laser beam in a given system.
The present invention addresses still further problems of the prior art and, in particular, the problems arising from the fact that laser systems which utilize diode pumped solid state (DPSS) lasers as well as diode pumped fiber lasers (DPFL) tend to require precise laser beam uniformity to enhance the specific process they are performing such as microvia drilling through multi-layered materials laser dicing of wafers, laser trimming or other laser materials processing. The output of a DPSS or DPFL laser sources tend to have TEMP00 single mode beams which take the form of a Gaussian beam profile. When processing, it is advantageous to transform this Gaussian beam profile by means of diffractive or holographic beam diffusers or shaping optics, into what is called a Flat Top beam with a uniform energy profile. However, as the DPSS laser parameters change (Pump Diode Current, Harmonic Crystal Shifting or Changes in Pulsing Frequency/Repetition Rate) the laser beam drifts laterally, ranging from a few microns to several hundred of microns. This lateral shift creates a problem for an industrial laser system on the production floor, since every time the beam laterally moves, the beam shaping optics generate a hot spot on side or direction that the beam shifts. At this point, the beam shaping optics and the beam delivery system must be realigned to the new laser beam centerline. This is a considerable problem when a production laser system has to change parameters several times a day or even several times per work shift of operation. The need is for a beam shaper that is simple to adjust to maintain the proper flat top profile, without having to realign the beam delivery system as well as an automatic compensation routine and sensor to monitor the displacement created by the changes in the laser parameters.
The present invention provides a solution to these and related problems of the prior art.